Increasing the flexibility with which an optical transport network may route Wavelength Division Multiplexed (WDM) traffic has traditionally increased the efficiency of the network. Reconfigurable Optical Add/Drop Multiplexers (ROADMs) have generally contributed to this increased routing flexibility by enabling traffic at the wavelength granularity to be selectively added or dropped at any node in the network. However, ROADMs employ fairly complex and expensive components to provide this flexible routing capability, meaning that ROAMDs prove cost-prohibitive in some contexts.
One such context relates to an aggregation network that efficiently transports the traffic of multiple services in a converged fashion. Rather than employing multiple different networks in parallel for transporting these different services (e.g., mobile, business, and residential services), a converged network transports those services together using the same network. A transport network that optically converges different services by transporting those services on different wavelengths would be advantageous, for a variety of reasons, but has heretofore been precluded in aggregation networks close to end-users by the high cost of the necessary hardware components (e.g., ROADMs).
Consequently, known aggregation networks converge different services using packet aggregation instead. While packet aggregation currently requires less hardware expense for converged transport, that expense will not scale equally as significant traffic increases are expected in the near future. Moreover, while packet aggregation suffices in many respects for realizing convergence, it may prove inefficient in implementation. Indeed, converting multiple services at the packet level involves significant complexity in order to accommodate the different packet requirements associated with the different services.
Some systems employ dense WDM at 25 GHz channel spacing, while most commercial systems for optical transport run over two unidirectional fibers. Also for access systems (e.g. passive optical networks), which typically use bidirectional fibers, so-called Ultra DWDM (UDWDM) has been proposed, with down to 3 GHz channel spacing. Such UDWDM systems typically run over power split optical networks without networking (wavelength switching) of the individual wavelengths. In very dense WDM, the stability of the wavelengths is critical since overlapping channels interfere with each other causing bit errors. For UDWDM, one solution is to create the very dense channels using a broadband modulator, thus not relying on the stability of individual lasers. In case the individual channels come from different ports or even different equipment, wavelength control of the individual lasers are required. This is typically done using combinations of temperature control and a wavelength reference, such as an etalon.
In order to make high utilization of the available fibers in a WDM system, high wavelength channel counts are attractive. While currently commercial DWDM systems offer up to 96 DWDM channels over a pair of fibers, using this channel spacing (50 GHz over the Central (C)-band) would result in 48 usable connections over a single fiber. Wth decreased channels spacing, such as going to 25 GHz interleaved channels, or even 25 GHz detuned channels, the number of usable connections may be increased to 96 and 192, respectively.
Wavelength control using etalons may offer stability in the order of +/−1.5 GHz, which may not be enough to assure satisfactory performance (e.g., Bit Error Rate (BER)) for the WDM, for example, with a channel spacing of 25 GHz. Moreover, absolute frequency stability as required by a local method comes at increasing cost as requirements become tighter. Current systems rely on various methods to solve the above mentioned problems. Examples of such current methods are centralized control with a wavelength reference, and local control using the Rayleigh backscatter of the transmitted signal. Centralized control methods suffer from high cost and slow response times. Local control based methods suffer from poor stability. Moreover, methods based on measuring optical power levels does not take into account cross-talks resulting from counter-propagating channels drifting close to each other.